Development of efficient methods for highly diastereoselective epoxidation of allylically substituted alkenes is of great importance, as their epoxides are versatile building blocks for organic synthesis as well as construction of biologically active natural products and chiral drugs.
trans-Epoxides of some allylic alkenes are known to be key synthetic intermediates/starting materials in the preparation of synthetically useful chiral 1,2-diamines [Demay, S.; Kotschy, A.; Knochel, P. Synthesis 2001, 863], conformationally rigid analogues of Carnitine [Hutchison, T. L.; Saeed, A.; Wolkowicz, P. E.; McMillin, J. B.; Brouillette, W. J. Bioorg. Med. Chem. 1999, 7, 1505], cyclopentane analogues of DNA [Ahn, D.-R.; Mosimann, M.; Leumann, C. J. J. Org. Chem. 2003, 68, 7693], core structure of Neocarzinostain antibiotics [Tanaka, H.; Yamada, H.; Matsuda, A.; Takahashi, T. Synlett. 1997, 381], biologically active natural products such as (+)-epiepoformin [Tachihara, T.; Kitahara, T. Tetrahedron 2003, 59, 1773], and several best-selling FDA approved HIV-protease inhibitors [Ghosh, A. K.; Bilcer, G.; Schiltz, G. Synthesis 2001, 15, 2203].
In addition, some trans-epoxides of cycloalkenes are fundamental structural units of biologically active natural products such as (+)-bromoxone [Block, O.; Klein, G.; Altenbach, H.-J.; Brauer, D. J. J. Org. Chem. 2000, 65, 716], (−)-cycloepoxydon [Li, C.; Pace, E. A.; Liang, M.-C.; Lobkovsky, E.; Gilmore, T. D.; Porco, J. A., Jr. J. Am. Chem. Soc. 2001, 123, 11308], and (+)-epoxyquinols A and B [Shoji, M.; Yamaguchi, J.; Kakeya, H.; Osada, H.; Hayashi, Y. Angew. Chem. Int. Ed. 2002, 41, 3192].
Significant advances have been achieved in cis-selective epoxidation of allylic alcohols through hydrogen bonding between their syn-directing hydroxyl group and oxidants. In general, highly cis-selective epoxides (cis:trans-epoxide ratio>20:1) could be conveniently obtained by using peracids such as m-chloroperoxybenzoic acid (m-CPBA) as oxidant [for reviews on highly cis-selective epoxidation, see: Hoveyda, A. H.; Evans, D. A.; Fu, G. C. Chem. Rev. 1993, 93, 1307. Adam, W.; Wirth, T. Acc. Chem. Res. 1999, 32, 703].
For epoxidation of allylically substituted alkenes without syn-directing groups, trans-epoxides would be obtained as major product through steric interaction between the substrates and the oxidants. However, the trans-selectivity (i.e., trans:cis-epoxide ratio) obtained by using the common oxidants such as m-CPBA and dioxiranes are generally low (i.e., trans:cis<20:1). Thus, the development of efficient methods for highly trans-selective epoxidation of allylic alkenes poses an important challenge in organic synthesis.
Recently, a systematic study on m-CPBA-mediated diastereoselective epoxidation of some selected N-protected 2-cyclohexen-1-ylamines has been reported [O'Brien, P.; Childs, A. C.; Ensor, G. J.; Hill, C. L.; Kirby, J. P.; Dearden, M. J.; Oxenford, S. J.; Rosser, C. M. Org. Lett. 2003, 5, 4955]. Dioxiranes (either isolated or generated in situ from ketones and oxone) have been reported as mild and efficient oxidants for trans-selective epoxidation of allylically substituted alkenes [see: Miyata, N.; Kurihara, M.; Ito, S.; Tsutsumi, N. Tetrahedron Lett. 1994, 35, 1577. Murray, R. W.; Singh, M.; Williams, B. L.; Moncrieff, H. M. Tetrahedron Lett. 1995, 36, 2437. Murray, R. W.; Singh, M.; Williams, B. L.; Moncrieff, H. M. J. Org. Chem. 1996, 61, 1830. Yang, D.; Jiao, G.-S.; Yip, Y.-C.; Wong, M.-K. J. Org. Chem. 1999, 64, 1635]. Methyltrioxorhenium (MTO) has been employed for diastereoselective epoxidation of cyclic allylic alkenes [Adam, W.; Mitchell, C. M.; Saha-Moller, C. R. Eur. J. Org. Chem. 1999, 785]. The main reason for their low trans-selectivities could be attributed to the weak/moderate steric interaction between the oxidants and the substrates.
Metalloporphyrin-catalyzed alkene epoxidation has been a subject of extensive investigation [Meunier, B. Chem. Rev. 1992, 92, 1411. Mansuy, D. Coord. Chem. Rev. 1993, 125, 129. Dolphin, D.; Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997, 30, 251].
As will be appreciated from the foregoing, metalloporphyrin catalysts have been used for the enantioselective epoxidations of alkenes.
Metalloporphyrins have been used as catalysts for regio- and shape-selective epoxidations of alkenes [Groves, J. T.; Nemo, T. E. J. Am. Chem. Soc. 1983, 105, 5786. Collman, J. P.; Brauman, J. I.; Meunier, B.; Hayashi, T.; Kodadek, T.; Raybuck, S. A. J. Am. Chem. Soc. 1985, 107, 2000. Groves, J. T.; Neumann, R. J. Am. Chem. Soc. 1987, 109, 5045. Collman, J. P.; Zhang, X.; Hembre, R. T.; Brauman, J. I. J. Am. Chem. Soc. 1990, 112, 5356.]
Chiral iron and manganese porphyrins have been employed for enantioselective alkene epoxidations [Groves, J. T.; Myers, R. S. J. Am. Chem. Soc. 1983, 105, 5791. Mansuy, D.; Battoni, P.; Renaud, J. P.; Guerin, P. J. Chem. Soc., Chem. Commun. 1985, 155. O'Malley, S. Kodadek, T. J. Am. Chem. Soc. 1989, 111, 9176. Grove, J. T.; Viski, P. J. Org. Chem. 1990, 55, 3628. Naruta, Y.; Tani, F.; Ishihara, N.; Maruyama, K. J. Am. Chem. Soc. 1991, 113, 6865. Halterman, R. L.; Jan, S.-T. J. Org. Chem. 1991, 56, 5253. Knoishi, K.; Oda, K.-I.; Nishida, K.; Aida, T.; Inoue, S. J. Am. Chem. Soc. 1992, 114, 1313. Collman, J. P.; Zhang, X.-M.; Lee, V. J.; Uffelman, E. S.; Brauman, J. I. Science 1993, 261, 1404. Collman, J. P.; Wang, Z.; Straumanis, A.; Quelquejeu, M. J. Am. Chem. Soc. 1999, 121, 460.]
Chiral ruthenium-porphyrins have been used as efficient catalysts for enantioselective epoxidation of alkenes [Gross, Z.; Ini, S. J. Org. Chem. 1997, 62, 5514. Berkessel, A.; Frauenkron, M. J. Chem. Soc., Perkin Trans. 1, 1997, 2265. Gross, Z.; Ini, S. Org. Lett. 1999, 1, 2077. Zhang, R.; Yu, W.-Y.; Lai, T.-S.; Che, C.-M. Chem. Commun. 1999, 409. Gross, Z.; Ini, S. Inorg. Chem. 1999, 38, 1446. Zhang, R.; Yu, W.-Y.; Wong, K.-Y.; Che, C.-M. J. Org. Chem. 2001, 66, 8145. Zhang, R.; Yu, W.-Y.; Sun, H.-Z.; Liu, W.-S.; Che, C.-M. Chem. Eur. J. 2002, 8, 2495.]
In addition, it has been reported that supported polyhalogenated metalloporphyrins are robust and recyclable catalysts for alkene epoxidations with exceptionally high turnover numbers [Groves, J. T.; Bonchio, M.; Carofiglio, T.; Shalyaev, K. J. Am. Chem. Soc. 1996, 118, 8961. Liu, C.-J.; Li, S.-G.; Pang, W.-Q.; Che, C.-M. Chem. Commun. 1997, 65. Che, C.-M.; Liu, C.-J.; Yu, W.-Y.; Li, S.-G. J. Org. Chem. 1998, 63, 7364. Che, C.-M.; Yu, X.-Q.; Huang, J.-S.; Yu, W.-Y. J. Am. Chem. Soc. 2000, 122, 5337. Che, C.-M.; Zhang, J.-L. Org. Lett. 2002, 4, 1911].
However, there is a paucity of reports of the use of metalloporphyrin catalysts for diastereoselective epoxidation of allylically substituted alkenes. It has been reported that high diastereoselectivity could be obtained in epoxidation of 3,4,6-tri-O-acetyl-D-glucal and 2-(Boc-amino)-1-phenylbut-3-ene using ruthenium-porphyrins as catalysts [Che, C.-M.; Liu, C.-J.; Yu, W.-Y.; Li, S.-G. J. Org. Chem. 1998, 63, 7364. Che, C.-M.; Yu, X.-Q.; Huang, J.-S.; Yu, W.-Y. J. Am. Chem. Soc. 2000, 122, 5337. Che, C.-M.; Zhang, J.-L. Org. Lett. 2002, 4, 1911]. There is exclusive formation of α-epoxide in the epoxidation of 3,4,6-tri-O-acetyl-D-glucal, which we believe could be attributed to the strong steric interaction between the bulky porphyrin ligand and the three O-acetyl groups on the substrate's ring. On the other hand, the threo-selectivity obtained in the epoxidation of 2-(Boc-amino)-1-phenylbut-3-ene appears to be due to the hydrogen bonding formation between the NHBoc group of the substrate and the metal oxo center of the porphyrin catalysts.
Iron porphyrins have been reported as catalysts in diastereoselective epoxidation of some hydroxy-protected acyclic chiral allylic alcohols, see: Adam, W.; Stegmann, V. R.; Saha-Moller, C. R. J. Am. Chem. Soc. 1999, 121, 1879. For these hydroxy-protected allylic alcohols, erythro selectivity was obtained in the epoxidation. The erythro selectivity could be attributed to steric effects between the substrates and the catalysts.
In view of the significance of trans-selective epoxides of allylically substituted alkenes in the synthesis of natural products and chiral drugs, there exists an urgent need to develop new, practical, and efficient methods for the synthesis of these synthetically useful epoxides.